A Pacific hagfish, Eptatretus stoutii, nicknamed the slime eel, as used in the study. These ancient vertebrates burrow into seafloor carcasses and eat their way out. Hagfish are the only vertebrates known to utilise external body surfaces for nutrient acquisition.
Credit: Wikimedia
CAMBRIDGE: The most ancient living vertebrate can absorb nutrients directly through its skin, biologists have revealed, reporting that this ability may provide clues about the evolution of the earliest vertebrates.
Most living vertebrates have impermeable skin and only absorb nutrients through specialised digestive systems. But the new findings, published in Proceedings of the Royal Society B, show that this is not the case for hagfish.
“Hagfish are thought to be the living animal that was first to branch off the main line of vertebrate evolution,” explained lead author Chris Glover at the University of Canterbury, New Zealand. “The key insight that this study provides is into the processes and trade-offs that may have shaped the evolution of physiological systems in vertebrates.”
The capacity and opportunity
Many aquatic invertebrates can absorb nutrients through their skin. This permeability means that their internal environments – acidity, salt concentrations and water levels – change to match the external environment.
In contrast, most vertebrates must keep their internal environment constant, and have evolved impermeable skin to prevent changes. Nutrient absorption is therefore restricted to the specialised surfaces of the internal digestive system.
Hagfish have a common ancestor with all other vertebrates – the ancestral vertebrate – but do not keep their internal environment constant, so do not need an impermeable skin. What is more, their feeding habits – burrowing into carcasses and eating their way out – expose them to high levels of dissolved nutrients. “Combined, these factors suggested they had the capacity and opportunity for nutrient absorption via the skin and gill,” said Glover.
Important model for vertebrate evolution
Glover and his colleagues confirmed this ability by demonstrating absorption of two radioactively labelled amino acids. Gills and skin tissue were removed from hagfish and exposed to the radioactive nutrients in a solution that flowed over the outer surfaces. After three hours, the levels of radioactivity in the tissue were measured.
Amino acids were found in the tissue of both the gills and the skin, showing that they had been absorbed through the external surface of the hagfish. This is the first time that such a mechanism of nutrient uptake has been seen in a vertebrate animal.
Uptake required sodium in the gills but did not in the skin, indicating distinct absorption pathways in each. Together with differing rates of uptake, this suggests that absorption occurs via specific transport pathways rather than simply by diffusion.
